A Multi-Input Multi-Output (MIMO) technique refers to a method capable of improving data transmission/reception efficiency using multiple transmit antennas and multiple receive antennas, instead of one transmit antenna and one receive antenna used up to now. Namely, MIMO is technology for increasing capacity or improving performance using multiple antennas at a transmitting end or receiving end of a wireless communication system. MIMO may also be called multiantenna technology.
To support MIMO transmission, precoding to appropriately distribute transmission information to respective antennas according to a channel state, etc. may be applied. Precoding may refer to mapping one or more layers to a plurality of antenna and such a mapping relationship may be expressed by a precoding matrix. A set of precoding matrices which can be used by the transmitting end for MIMO transmission may be predefined and shared among the transmitting end and the receiving end. This set of precoding matrices is referred to as a precoding codebook or simply a codebook.
In a legacy 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system, a maximum of four transmit (4Tx) antennas is supported for downlink transmission and 2Tx antennas and a codebook for transmission of 2Tx antennas are defined.